The present invention relates to control systems and, more particularly, to trip controllers. A major objective of the present invention is a trip controller that provides reliable interlocking in response to pressure changes in a integrated circuit wafer processor.
Certain wafer processors involve processing at subatmospheric pressures. The seals that allow the reduced pressures to be achieved must be released to insert wafers before processing into a chamber and to withdraw the processed wafers from the chamber after processing.
Highly automated wafer processors require a sophisticated interlock system to prevent damaging errors. For example, chamber opening should be precluded during vacuum, while purge gas flow should be prohibited once atmospheric pressure is achieved. Accordingly, a trip controller can be used to prohibit or allow certain operations to occur, depending on whether pressure within the chamber is at or below atmosphere.
A conventional trip controller includes a pressure transducer that outputs a voltage as a function of sensed differential pressure between the interior and exterior of a processing chamber. A comparator compares the transducer output voltage with a reference voltage corresponding to atmospheric pressure. The output of the comparator is used to enable/disable the functions associated with the pressure interlock.
Reliability can be a problem with such a trip controller. The signal output by the transducer at a given pressure differential can drift over time, causing a trip to occur above or below the desired pressure. If, for example, the trip controller indicates atmospheric pressure has been reached while the chamber is actually under vacuum, the chamber may be opened prematurely. In this case, the pressure differential can cause turbulence that can stir up contamination, adversely affecting wafer quality. If the trip controller is set too high, the interlock can prevent chamber opening even at atmosphere.
This problem can be addressed by frequent recalibration. The trigger point can be tested by comparison with a digital pressure meter. The trip controller transducer can be adjusted so that its indication of atmosphere matches that of the external pressure meter. However, the calibration can be inconvenient, interrupting processing and requiring skills quite different from those required merely to run the wafer processing equipment.
Furthermore, even a properly calibrated system can have reliability problems. As the pressure of a chamber is increased toward atmosphere, a purge gas can be applied to remove residual reactant gases. The purge gas provides a slightly positive pressure, causing an early indication of atmospheric pressure. If the chamber is thus prematurely opened, the internal pressure can drop as the purge gas is released, causing the interlock to reverse. Thus, there can be oscillating indications, resulting in minor interlock instabilities. What is needed is a trip controller that provides for reliable operation with minimal requirements for calibration.